LONDON—The crude oil price rally could be short-lived and global oil demand will be weaker than expected this year and next, the International Energy Agency said Tuesday.

In its closely watched monthly oil report, the IEA cut its crude demand growth outlook by 100,000 barrels a day for 2017 and 2018. The agency now expects demand to grow by 1.5 million barrels a day this year and 1.3 million barrels a day next year.

The IEA noted that oil prices have risen roughly 20% since early September—with Brent crude sustaining gains above $60 a barrel in recent weeks—on the back of supply disruptions and geopolitical tensions in the Middle East. But if those problems prove temporary, a “fresh look at the fundamentals” would likely show the “market balance in 2018 does not look as tight as some would like and there is not in fact a ‘new normal.’”

The IEA’s findings stand in contrast to that of the Organization of the Petroleum Exporting Countries, which released its monthly oil report Monday. OPEC raised its forecasts for global oil demand for this year and next, touting increased market rebalancing and stability.

The two reports come ahead of a highly anticipated OPEC meeting in Vienna on Nov. 30, at which the cartel and some other major producers are set to debate whether to extend an agreement to rein in production.

OPEC and 10 members outside the cartel, including Russia, first agreed a year ago to cut almost 2% of global oil production in an effort to tackle a glut and boost prices. Analysts widely expect the participants to extend the deal through the end of 2018, after it expires in March.

The IEA on Tuesday said that global oil supply had risen in October by 100,000 barrels a day to 97.5 million barrels a day, driven by non-OPEC production in the North Sea and Mexico.

However, OPEC output fell by 80,000 barrels a day in October to 32.53 million barrels a day, as a result of lower production in Iraq, Algeria and Nigeria—the lowest level since May, according to the IEA. The figure was roughly in line with OPEC’s own estimate of 32.59 million barrels a day.

Commercial petroleum stocks in the Organization for Economic Cooperation and Development—a group of industrialized, oil-consuming nations, including the U. S.—fell below 3 billion barrels in September, for the first time in two years, the IEA said.

The agency this week also released its annual World Energy Outlook, in which it said global oil demand would not peak before 2040, although demand growth should slow “considerably” after 2025 amid greater fuel efficiency and electrification.

IEA Executive Director Fatih Birol said even as penetration of electric vehicles drags down oil demand for passenger vehicles, global oil demand should continue to be strong through 2040 for commercial trucks, planes, ships and petrochemicals.